This invention relates generally to coated fiber strands for reinforcing composites and, more specifically, to coated fiber strands that are compatible with a matrix material that the strands are incorporated into.
In thermosetting molding operations, good xe2x80x9cwet-throughxe2x80x9d (penetration of a polymeric matrix material through the mat or fabric) and xe2x80x9cwet-outxe2x80x9d (penetration of a polymeric matrix material through the individual bundles or strands of fibers in the mat or fabric) properties are desirable. In contrast, good dispersion properties (i.e., good distribution properties of fibers within a thermoplastic material) are of predominant concern in typical thermoplastic molding operations.
In the case of composites or laminates formed from fiber strands woven into fabrics, in addition to providing good wet-through and good wet-out properties of the strands, it is desirable that the coating on the surfaces of the fibers strands protect the fibers from abrasion during processing, provide for good weavability, particularly on air-jet looms and be compatible with the polymeric matrix material into which the fiber strands are incorporated. However, many sizing components are not compatible with the polymeric matrix materials and can adversely affect adhesion between the glass fibers and the polymeric matrix material. For example, starch, which is a commonly used sizing component for textile fibers, is generally not compatible with polymeric matrix material. As a result, these incompatible materials must be removed from the fabric prior to impregnation with the polymeric matrix material.
The removal of such non-resin compatible sizing materials, i.e., de-greasing or de-oiling the fabric, can be accomplished through a variety of techniques. The removal of these non-resin compatible sizing materials is most commonly accomplished by exposing the woven fabric to elevated temperatures for extended periods of time to thermally decompose the sizing(s) (commonly referred to as heat-cleaning). A conventional heat-cleaning process involves heating the fabric at 380xc2x0 C. for 60-80 hours. However, such heat cleaning steps are detrimental to the strength of the glass fibers, are not always completely successful in removing the incompatible materials and can further contaminate the fabric with sizing decomposition products. Other methods of removing sizing materials have been tried, such as water washing and/or chemical removal. However, such methods generally require significant reformulation of the sizing compositions for compatibility with such water washing and/or chemical removal operations and are generally not as effective as heat-cleaning in removing all the incompatible sizing materials.
In addition, since the weaving process can be quite abrasive to the fiber glass yarns, those yarns used as warp yarns are typically subjected to a secondary coating step prior to weaving, commonly referred to as xe2x80x9cslashingxe2x80x9d, to coat the warp yarns with an abrasion resistance coating (commonly referred to as a xe2x80x9cslashing sizexe2x80x9d) to help minimize abrasive wear of the glass fibers. The slashing size is generally applied over the primary size that was previously applied to the glass fibers during the fiber forming operation. However, since typical slashing sizes are also not generally compatible with the polymeric matrix materials, they too must be removed from the woven fabric prior to its incorporation into the resin.
Furthermore, to improve adhesion between the de-greased or de-oiled fabric and the polymeric resin, a finishing size, typically a silane coupling agent and water, is applied to the fabric to re-coat the glass fibers in yet another processing step (commonly called xe2x80x9cfinishingxe2x80x9d).
All of these non-value added processing steps: slashing, de-greasing or de-oiling, and finishing, increase fabric production cycle time and cost. Additionally, they generally require significant investment in capital equipment and labor. Moreover, the added handling of the fabric associated with these processing steps can lead to fabric damage and decreased quality.
Efforts have been directed toward improving the efficiency or effectiveness of some of these processing steps. There nevertheless remains a need for coatings that can accomplish one or more of the following: inhibit abrasion and breakage of glass fibers; be compatible with a wide variety of matrix materials; and provide for good wet-out and wet-through by the matrix material. In addition, it would be particularly advantageous if the coatings were compatible with modern air-jet weaving equipment to increase productivity. Furthermore, it would be advantageous to eliminate the non-value added processing steps in a fabric forming operation while maintaining the fabric quality required for electronic support applications and providing for good laminate properties.